The present invention relates to a subband encoding and decoding system, and more particularly to a subband decoding and decoding system preferably used for compressing a digital signal.
A representative example of a conventional subband encoding system is a MPEG1 audio system. FIG. 40 is a block diagram showing a conventional MPEG1 audio layer encoding system. An encoding input digital signal s101 of sampling frequency fs, which is an input signal entered into this encoding system, is supplied to a band splitting section a101. The band splitting section a101 splits the input signal s101 for encoder into a total of k band components successive in an entire frequency zone ranging from 0 to a Nyquist frequency (fs/2) of the encoder input signal s101, where xe2x80x9ckxe2x80x9d is an arbitrary integer. The band splitting section a101 outputs each subband signal s102 of k split bands. For example, MPEG1 audio is based on a uniform band width slpitting of k=32. However, instead of using the uniform splitting, it is possible to adopt a non-uniform splitting depending on an individual filter arrangement, provided that each of k split band widths is a predetermined fixed value.
Furthermore, in the MPEG1 audio layer, each subband signal is down-sampled into a baseband signal by using a sort of frequency modulation. Meanwhile, while maintaining time synchronization with the band splitting section a101, a time-frequency converting section a102 performs a time-frequency conversion on the encoding input digital signal s101, wherein time window curtain is applied to w samples each having a unit sample length equivalent to a reciprocal (1/fs) of the sampling frequency. The time-frequency converting section a102 outputs frequency information s103 as a result of the time-frequency conversion. A time window length xe2x80x9cwxe2x80x9d used for the time-frequency conversion is obtained according to a frequency resolution xe2x80x9cfrxe2x80x9d required for the frequency information s103.
W=(1/fr)/(1/fs)
According to the MPEG1 audio layer, a fast Fourier transform technique is used for time-frequency conversion. The value xe2x80x9cwxe2x80x9d is defined as a minimum value 2w satisfying the required frequency resolution xe2x80x9cfr.xe2x80x9d Furthermore, considering time continuity, an appropriate overlap zone is provided between two consecutive time windows.
A frequency analyzing section a103 calculates a bit allocation number for each of k split bands to produce bit allocation information s104, by using a conventionally known auditory masking based on a psychoacoustic model, during a time length of a time window excluding the overlap zone used in the time-frequency converting section a102. The time length of a time window excluding the overlap zone is a unit time length of frame. An encoding section a104 produces a scale factor of each split band with reference to a maximum amplitude value per unit frame length of each subband signal s102. Based on the obtained scale factor of each split band, the amplitude of each subband signal s102 is normalized. Subsequently, requantization for each split is performed band based on the bit allocation information s104. The encoding section a104 forms a bit stream including the requantized sample, the bit allocation information, the scale factor and a frame sync information. Thus, the encoding section a104 produces a coded output signal s105.
FIG. 41 is a block diagram showing a conventional MPEG1 audio layer decoding system. A decoder input signal s106, which is a coded signal produced from the encoding system, is entered into the decoding system. A frame analyzing section a105 detects a frame, bit allocation information, and scale factor contained in the decoder input signal s106, thereby producing frame analysis information s107. A decoding section a106 performs the decoding processing for each split band based on the frame analysis information s107 to output a subband signal s108. Subsequently, a band combining section a107 combines the subband signals s108 to output a decoded output signal s109. To prevent any deterioration of information through the encoding-decoding processing, the condition required for the band combining device is to establish perfect reconstruction conditions matching with the band splitting section a101 of the encoding system. A conventionally known technique using QMF provides a filter arrangement satisfying such perfect reconstruction conditions.
However, the conventional MPEG subband encoding system performs the scale factor information and bit allocation information producing processing as well as the requantization processing for each of k split bands, and then constructs a frame with reference to the obtained information. This significantly increases a processing amount in the encoding processing and also increases a bit rate.
Furthermore, the conventional MPEG subband encoding system performs the compression of information based on the psychoacoustic model. Thus, the time-frequency conversion and the signal analysis in frequency regions are inevitable. To realize highly efficient compression without causing deterioration of information, it is necessary to maintain sufficient frequency resolution. To realize this, the frequency conversion requires a window curtain applied to a sufficiently long time sample. Regarding the delay time in the subband encoding and decoding processing, a frame length is determined based on a sample number required for the window curtain processing. This frame length serves as a unit length for performing each of the encoding processing, the decoding processing, and the buffering processing. Thus, the delay time depends on a processing time per frame length caused in each processing and a group delay of the band split filter. Thus, the processing delay time necessarily increases when to realize high sound quality and high compression rate.
Furthermore, the conventional MPEG subband encoding system requires a great processing amount for the frequency analysis and the bit allocation processing.
Moreover, when the conventional MPEG subband encoding system is used for radio transmission, it is necessary to add the sync word generating processing and the sync word detecting processing for performing a clock sync acquisition of a receiving system and a synchronization of a radio frame. To reduce errors caused in a transmission path, it is necessary to add the error correction processing separately. Accordingly, the processing delay time of an overall system further increases due to a buffering time in each processing etc. The separately added error correction processing is performed without considering characteristics of each information in the subband encoding processing. Thus, even in a preferable condition where a burst error is acceptable or a bit error rate during a long time period is relatively low, a fatal error may occur at an application level.
To solve the above-described conventional problems, the present invention has an object to provide a subband encoding and decoding system capable of reducing both the encoding processing amount and the encoding bit rate.
In order to accomplish this and other related objects, the present invention provides a subband encoding system comprising a band splitting means for implementing a band splitting on an encoder input signal to produce subband signals, a scale factor producing means for producing scale factor information in accordance with a signal output level of each subband signal, a bit allocation producing means for calculating bit allocation information based on the scale factor information, a requantizing means for implementing requantization based on the subband signal, the scale factor information and the bit allocation information, thereby outputting a requantized output signal, a frame constructing means for constructing a coded frame based on the requantized output signal and the scale factor information, thereby outputting a coded output signal, and a limiting means for limiting a total number of split bands of the requantized output signal based on an audible upper limit frequency.
With this arrangement, the bit allocation is calculated based on the scale factor without requiring the frequency analysis. The subband encoding is performed based on the information representing a change of the scale factor relative to a scale factor of an immediately previous frame. The total number of split bands is limited based on the audible upper limit frequency. Both the encoding processing amount and the encoding bit rate can be reduced.
Preferably, a group scale factor information producing means is provided for producing group scale factor information based on the scale factor information, the group scale factor information being obtained as a representative value of each split band group. The bit allocation producing means calculates the bit allocation information based on the group scale factor information, the requantizing means requantizes the subband signal based on the group scale factor information, thereby outputting the requantized output signal. And, the frame constructing means outputs the coded output signal based on the group scale factor information and the requantized output signal.
With this arrangement, the subband encoding is performed based on the group scale factor, thereby reducing both the encoding processing amount and the encoding bit rate.
Preferably, the subband encoding system further comprises a means for determining an upper limit split band number based on a minimum integer satisfying the following formula
((encoder input signal sampling frequency/2)/(total number of split bands)xc3x97(upper limit split band number ))xe2x89xa7(upper limit frequency on application)
and then for setting an encoding processible upper limit frequency.
With this arrangement, it becomes possible to perform a real time output operation by constituting a frame having a time length equal to a minimum unit or twice of the same, the minimum unit being required for expressing a minimum frequency of the band splitting in the subband encoding.
Preferably, the bit allocation producing means obtains a ratio of the scale factor information in each split band to a minimum value of an audible smallest value in a corresponding split band group, and produces the bit allocation information based on an energy ratio of each of all split bands considering the audible smallest value.
With this arrangement, in the subband encoding, the bit allocation is efficiently produced by using the energy ratio determined based on the scale factor information of each split band and the audible smallest value which is the conventionally known characteristics. Thus, the calculation amount required for the bit allocation processing can be reduced.
Preferably, the subband encoding system further comprises a means for equalizing an encoding frame length with a transmission frame length, a means for putting additional transmission information during an encoding operation, the additional transmission information including a sync word, a means for performing an error-correction encoding processing during an encoding operation, and a means for performing interleave processing during construction of a coded frame.
With this arrangement, the sync acquisition processing, the error correction processing, and the interleave processing are performed during the frame construction of the subband encoding used for the radio transmission. Thus, the processing delay time of an overall system can be reduced.
Preferably, the error-correction encoding processing is performed by assigning different error-correction capabilities to respective information constituting a coded frame considering characteristics of encoder error robustness.
With this arrangement, in the subband encoding processing used for radio transmission, it becomes possible to differentiate the error correction processing in accordance with importance of the frame information at an application level. Thus, it becomes possible to prevent the occurrence of fatal error at an application level in a condition where a bit error rate during a long time period is relatively low.
Preferred embodiments of the present invention provide a subband encoding system comprising: band splitting means for implementing a band splitting on an encoder input signal to produce subband signals; scale factor producing means for producing scale factor information in accordance with a signal output level of each subband signal; bit allocation producing means for calculating bit allocation information based on the scale factor information; requantizing means for implementing requantization based on the subband signal, the scale factor information and the bit allocation information, thereby outputting a requantized output signal; frame constructing means for constructing a coded frame based on the requantized output signal and the scale factor information, thereby outputting a coded output signal; and limiting means for limiting a total number of split bands of the requantized output signal based on an audible upper limit frequency. Limiting the total number of the split bands makes it possible to reduce the encoding bit rate and the encoding processing amount.
Furthermore, preferred embodiments of the present invention provide a subband decoding system comprising: frame analyzing means for receiving a subband coded signal as a decoder input signal and for detecting a requantized signal and scale factor information from the received subband coded signal while maintaining frame synchronization; bit allocation producing means for producing bit allocation information based on the scale factor information; subband signal producing means for producing a subband signal from the requantized signal based on the scale factor information and the bit allocation information; and band combining means for combining subband signals produced from the subband signal producing means. Limiting the total number of the split bands makes it possible to reduce the decoding processing amount.
A preferred embodiment of the present invention provides the subband encoding system further comprising group scale factor information producing means for producing group scale factor information based on the scale factor information, the group scale factor information being obtained as a representative value of each split band group, wherein the bit allocation producing means calculates the bit allocation information based on the group scale factor information, the requantizing means requantizes the subband signal based on the group scale factor information, thereby outputting the requantized output signal, and the frame constructing means outputs the coded output signal based on the group scale factor information and the requantized output signal. Grouping the split bands and producing a representative scale factor information makes it possible to reduce the encoding bit rate and the encoding processing amount.
According to a preferred embodiment of the present invention, the frame analyzing means receives the subband coded signal as a decoder input signal and detects the requantized signal and group scale factor information from the received subband coded signal while maintaining frame synchronization, the bit allocation producing means produces the bit allocation information based on the group scale factor information, and the subband signal producing means produces the subband signal from the requantized signal based on the group scale factor information and the bit allocation information. Grouping the split bands and producing a representative scale factor information makes it possible to reduce the decoding processing amount and the encoding bit rate.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for determining an upper limit split band number based on a minimum integer satisfying the following formula
((encoder input signal sampling frequency/2)/(total number of split bands)xc3x97(upper limit split band number ))xe2x89xa7(upper limit frequency on application)
and then for setting an encoding processible upper limit frequency.
Thus, it becomes possible to set the minimum upper limit frequency satisfying a request on application.
According to a preferred embodiment of the present invention, the group scale factor information producing means produces the group scale factor information for each of 6-20 split band groups when encoding processing is performed for 32 split bands ranging from 0 to a frequency equivalent to (sampling frequency)/2.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for setting an encoding frame length equal to (total number of split bands)/(sampling frequency number). Thus, the processing delay time in the encoding processing is so reduced that the real time output operation is realized.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for setting an encoding frame length equal to (total number of split bands)xc3x972/(sampling frequency number). Thus, the processing delay time in the encoding processing is so reduced that the real time output operation is realized.
According to a preferred embodiment of the present invention, the bit allocation producing means obtains a ratio of the scale factor information in each split band to a minimum value of an audible smallest value in a corresponding split band group, and produces the bit allocation information based on an energy ratio of each of all split bands considering the audible smallest value. Thus, a calculation amount required for the bit allocation processing can be reduced.
According to a preferred embodiment of the present invention, the bit allocation producing means obtains a ratio of the scale factor information in each split band to an average value of an audible smallest value in a corresponding split band group, and produces the bit allocation information based on an energy ratio of each of all split bands considering the audible smallest value. Thus, a calculation amount required for the bit allocation processing can be reduced.
According to a preferred embodiment of the present invention, the bit allocation producing means converts the bit allocation information into a comparable integer for each band, wherein remaining allocatable bits are allocated one by one to appropriate split bands according to a ranking order of the split bands determined based on largeness of a numerical value in the digits lower than a decimal point in their energy ratios. Thus, the encoding bits can be effectively utilized.
According to a preferred embodiment of the present invention, the bit allocation producing means produces the bit allocation information by multiplying a weighting factor of each frequency region. Thus, the sound quality in the encoding processing can be improved.
According to a preferred embodiment of the present invention, the bit allocation producing means produces the bit allocation information by multiplying a weighting factor of the scale factor information of each split band. Thus, the sound quality in the encoding processing can be improved.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for equalizing an encoding frame length with a transmission frame length, and a means for putting additional transmission information during an encoding operation, the additional transmission information including a sync word. Thus, a buffering time and a processing amount required for the transmission path encoding and decoding processing can be shortened and the encoding bit rate can be reduced when this subband encoding system is used for radio transmission.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for transmitting a sync-dedicated frame at predetermined time intervals, the sync-dedicated frame consisting of sync acquisition words only. Thus, even when the scale factor information is deteriorated due to transmission error when this subband encoding system is used for radio transmission, a reset time required for restoring the scale factor information can be shortened.
A preferred embodiment of the present invention provides the subband decoding system further comprising a means for performing mute processing applied to a sync-dedicated frame which appears at predetermined time intervals and consists of sync acquisition words only, and a means for performing interpolation of data applied to a digital signal in a decoding processing section. Thus, even if a data blank of one frame occurs during the encoding and decoding processing, it becomes possible to prevent such a data blank from being detected at a user side.
A preferred embodiment of the present invention provides the subband decoding system further comprising a means for performing mute processing applied to a sync-dedicated frame which appears at predetermined time intervals and consists of sync acquisition words only, and a means for performing interpolation of data applied to an analog signal in a decoding processing section. Thus, even if a data blank of one frame occurs during the encoding and decoding processing, it becomes possible to prevent such a data blank from being detected at a user side.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for performing an error-correction encoding processing during an encoding operation. Thus, the transmission error can be reduced when this subband encoding system is used for radio transmission.
According to a preferred embodiment of the present invention, the error-correction encoding processing is performed by assigning different error-correction capabilities to respective information constituting a coded frame considering characteristics of encoder error robustness. Thus, the transmission error can be reduced when this subband encoding system is used for radio transmission.
According to a preferred embodiment of the present invention, the error-correction encoding processing is performed by using BCH codes. Thus, the transmission error can be reduced when this subband encoding system is used for radio transmission.
According to a preferred embodiment of the present invention, the error-correction encoding processing is performed by using a convolutional coding. Thus, the transmission error can be reduced when this subband encoding system is used for radio transmission.
According to a preferred embodiment of the present invention, the error-correction encoding processing is performed by assigning different types of error correction codes to respective information constituting a coded frame considering characteristics of encoder error robustness. Thus, the transmission error can be reduced when this subband encoding system is used for radio transmission.
According to a preferred embodiment of the present invention, the different types of error correction codes are a combination of BCH and convolutional codes. Thus, the transmission error can be reduced when this subband encoding system is used for radio transmission.
According to a preferred embodiment of the present invention, the error-correction encoding processing is performed for part of information constituting a coded frame in response to importance of each information. Thus, the encoding bit rate can be reduced.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for rearranging requantized output signals considering influence of encoding error. Thus, it becomes possible to eliminate deterioration due to encoding error at the application level.
A preferred embodiment of the present invention provides the subband decoding system further comprising a means for performing mute processing applied to a frame having an error bit number not smaller than a predetermined threshold, and a means for performing interpolation of data applied to a digital signal in a decoding processing section. Thus, even if an encoding error occurs during radio transmission, it becomes possible to interpolate it without being detected at a user side.
A preferred embodiment of the present invention provides the subband decoding system further comprising a means for performing mute processing applied to a frame having an error bit number not smaller than a predetermined threshold, and a means for performing interpolation of data applied to an analog signal in a decoding processing section. Thus, even if an encoding error occurs during radio transmission, it becomes possible to interpolate it without being detected at a user side.
A preferred embodiment of the present invention provides the subband encoding system further comprising a means for performing interleave processing during construction of a coded frame. Thus, when this subband encoding system is used for radio transmission, a burst-like transmission error can be reduced.
A preferred embodiment of the present invention provides the subband decoding system further comprising a means for performing de-interleave processing during analysis of a transmitted frame. Thus, when this subband decoding system is used for radio transmission, a burst-like transmission error can be reduced.